Once fumigated, produce has a shelf life of at most ten days to two weeks. It is therefore desirable to fumigate the produce close to the place where it will be ultimately sold. For imported produce, this place is typically inside a dock building located at a port of entry. Pallets of the produce are unloaded from a ship and placed or staged on the floor of the dock building.
Known methods of fumigating produce involve covering the pallets of produce with a polyethylene or plastic impregnated tarpaulin and then introducing a gas, e.g., methyl bromide, under the tarp to kill live insect infestations. Workers climb on top of the pallets or use poles to pull the tarp over a group of pallets. Because it is necessary for the gas to circulate freely on all sides and the top of the pallets, the tarp cannot lay flat on the top of the pallets. The United States Department of Agriculture requires that a space of about two feet be provided on the top of the pallets, and at least one foot on their sides to allow room for the placement of fans to facilitate the circulation of the fumigation gas. The fans may be tied down with rope to prevent them from moving. A hose for introducing the gas is secured to the top of selected fans. The volume of space under the tarp, i.e., the total cubic feet, determines how many fans and points of introduction are required.
One method for maintaining the required space on the top of the pallets is to erect wood frames in the shape of a "T" at each corner and attach rope between each "T" to create a web of support for the tarp. The side space is maintained by draping the tarp outward as it contacts the floor of the dock building and securing it in that position by placing vinyl tubes (approximately 6" in diameter and 4' long) filled with sand on top of the edge of the tarp. At least 2 feet of excess tarp remains beyond the `sand tubes`.
The steps involved in pulling the tarp over the pallets, setting up the fans, attaching the introduction hoses to the fans, handling hundreds of tubes of sand, and dismantling and storing all of the above equipment, and repeating the process upon completion, require many labor intensive hours.
Improvements to the conventional method of fumigating produce include permanently suspending the tarpaulin from an overhead frame and lowering the frame over the pallets as needed. Such a frame may be made from welded trusses to form a disassemblable one piece unit of varying dimensions. The typical frame size may be 50 feet long by 250 feet wide, but may be larger or smaller depending upon the spacing between support columns in the dock building. To the underside of the frame a tarpaulin is attached which has been manufactured with grommets, i.e., tabs and eyelets, specific to the frame it hangs from.
The frame may be raised and lowered by a system of cables and pulleys located above the unit which are attached to exposed barjoists on the roof of the dock building. The cables are connected to hand-operated winchs mounted on the support columns in the dock building. Workers simultaneously crank the winches to lower or raise the frame to a height above the pallets that allows the required 2 feet of space between the tarp and pallets of produce.
A typical tent may be lifted by twelve active pickup points. Between each of these points two counterweight sandbags are deployed to offset the self-weight of the frame and the polyethylene enclosure. Each truss section of the frame spans approximately fifty feet with two mid-point sandbags. These tend to become hung up from friction and pull at an angle to prevent the sandbag from lowering onto the polyethylene enclosure as it is elevated.
A difficult and time-consuming job associated with operating such a system is gathering up the tarp which overhangs on the sides of the frame before the frame is raised. This is necessary because the typical dock building height of about twenty-two feet is limited to about eighteen feet by overhead piping, electrical conduits, etc. When the support frame is raised to the maximum height permitted by these obstructions the tarp hangs down low enough to catch the tops of forklift trucks as they move the pallets of produce in and out of the tent area.
One known way of gathering up the tarp on the sides is to run horizontal plastic pipes through double-thickness portions of the tent walls which form pockets. These pockets can be located at approximately one-third and two-thirds of the wall height and are tied up to the top framing with rope every ten feet or so. Hand-pulled ropes raise the pockets and thus gather the sides of the tent. This method is also time-consuming, however, because it requires numerous tie-up procedures in order to completely raise the tent, e.g., usually at ten to twenty foot intervals around the tent perimeter.